Ethylene is one of the most widely produced petrochemicals in the world. Most ethylene is produced through the cracking of hydrocarbons. Acetylene is a byproduct of ethylene production processes and acts as a poison to the catalysts used for making polyethylene out of the ethylene product. In addition, acetylene can form metal acetylides, which are explosive contaminants. Polymer-grade ethylene product should contain no more than 1 ppm of acetylene. Trace acetylene removal by selective hydrogenation is practiced commercially but is a significant challenge to the ethylene producer and catalyst manufacturer. This is due to the low acetylene concentration in the reactor effluent and the necessity to convert nearly 100% of the acetylene without decreasing ethylene yields due to the conversion of ethylene to ethane. Ethylene is a valuable feedstock for several chemical processes and it is advantageous to selectively reduce acetylene to ethylene. In the reduction of acetylene, ethylene selectivity and the useful life of the catalyst are important variables when choosing a catalyst.
Alkynes are easily chemically reduced to alkanes by the addition of H2 over a metal catalyst. The reaction takes place in steps through an alkene intermediate. It is possible to selectively terminate the chemical reduction of acetylene at ethylene, prior to further chemical reduction to ethane, by controlling the selectivity of the catalyst. Al2O3 is often used as a support for metal catalysts and possesses the ability to function both as a Lewis acid and as a Lewis base. Ruthenium, while possessing the greatest activity of the platinum group metals, does have drawbacks when used as a supported metal catalyst in hydrocarbon gas streams containing acetylene.
Platinum group metals, e.g. Ruthenium, used in catalysts intended for the reduction of acetylene can convert significant fractions of acetylene into ethane through hydrogenation of ethylene.
In addition to hydrocarbons, an off-gas stream often contains nitric oxides, oxygen, sulfur, and other impurities. Most selective acetylene hydrogenation operations at the commercial scale use Pd-based catalysts. The Pd-based catalysts have high activity and selectivity for selective hydrogenation of acetylene and dienes; but they are very sensitive to sulfur and some other catalyst poisons. Moreover, the Pd-based catalysts are not known to be particularly effective for removal of nitric oxides and oxygen.
Nickel catalysts have also been used in selective hydrogenation of acetylene and dienes. Nickel catalysts are resistant to sulfur poisoning, but are not selective toward hydrogenation of acetylene. Most commonly, while acetylene is removed, significant amounts of olefins are also hydrogenated to saturated hydrocarbons. Nickel-based catalysts also tend to form nickel carbonyl when the carbon monoxide level is high in the feed gas stream, particularly at low temperatures. Nickel carbonyl is a highly volatile, highly toxic substance which can deposit in downstream equipment and pose a significant safety hazard to workers.